Since its release, the iPhone hasn't allowed you to change the default apps for calling and messaging. However, with iOS 18.2, this option has become available, and WhatsApp was the first popular messenger to implement such support.
This is reported by 9to5Mac.
Thus, by transferring calls and messages to WhatsApp in the iOS settings, this messenger will respond to the corresponding actions instead of native Apple applications. That is, for example, by clicking on a phone number, say, on some website, the smartphone will offer to call via WhatsApp, and not the standard Phone.app.
It is worth noting that previously only Zoom could be set as the default calling app. However, the general idea of this service is still different from popular messengers.
Among other things, users can also change default email apps, call filters, browsers, password managers, and keyboards. These changes are thanks to previously adopted European Union regulations.
If you still don't have this option in your settings, you should check for installed updates for the operating system and the application itself.